Remote clock synchronization system

ABSTRACT

A system for the precise synchronization of high frequency electronic clocks by providing each station other than the master station with a tracking circuit. The tracking circuit consists of an early gate generated just prior to the arrival of the synchronizing pulse from the master station and a late gate generated just after the synchronizing pulse is received from the master station. If the synchronizing pulse received from the master station is either late or early the clock will either speed up or slow down so that it will be in synchronization with the synchronizing pulses received from the master station.

United States Patent 3,801,981

Al ers Apr. 2, 1974 I REMOTE CLOCK SYNCHRONIZATION SYSTEM PrimaryExaminer-Stephen C. Bentley Inventor: Frederick C. p Riverside Att0rney,A gen t, or Firm-R. S. Scrascra; G. J. Rubens;

. T. M. Phllhps Calif.

{73] Assignee: The United States of America as represented by theSecretary of the [57] ABSTRACT Navy, Washington, DC. A system for theprecise synchronization of high frequency electronic clocks by providingeach station [22] FlIed' 1971 other than the master station with atracking circuit. [211 App]. No.: 136,989 The tracking circuit consistsof an early gate generated just prior to the arrival of thesynchronizing pulse from the master station and a late gate generatedjust [5?] US. Cll. "601143475? after the Synchronizing pulse is receivedfrom the .ius. "i; t

{7 5 ter Station. the Synchronizing pulse received from 1 le 0 Cal C themaster stat n is e e late or e y t e ock either speed up or slow down sothat it will be in syn- I56I References Clted chronization with thesynchronizing pulses received UNITED STATES PATENTS from the masterstation.

3,250,896 5/1966 Perinson et a]. 343/75 3,440,652 4/1969 Bates ct al.343/75 6 Clam, 4 D'awmg F'gures svncmaomzms PULSE FROM MASTER STATION Fwas 5a ozrscnou fii'aififli" V V (54 (56 BISTABLE BISTABLEMULTIVI1BRATOR MULTIVIBRATOR 2 ONT Torr ONT OFF ADDITIONAL ADDITIONALcouNTI-IR 1- COUNTER 2 30 I t I 32 I HIGH I FREQUENCY REACTANCEOSCILLATOR IT I I I 34 I I I I RESET eEZ FIi roR w? i I I I CLOCK PULSESI I 46 TARGET RADAR OR OTHER EVENTH EVENT STOP TIMING EVENT TIME 0ARRIVAL DATA TRANSMITTER BEACOTNOPULSE OTHER STATIONS PAIENTEIJIPII 2I974 3.801.981

SHEET 2 BF 3 MASTER CLOCK CLOCK PULSES START /PULSES RECEIVED BEACONPULSE FROM TIME INTERVAL REMOTE STATION STOP COUNTER gla d MEMORY UNIT NV S DIGITAL I6 ADD CIRCUIT (dd START 20 DIGITAL SET PRESET gi J gCOUNTER 2o (Nd s '4 FIG.

SYNCH PULSE FOR REMOTE STATION INVENTOR. FREDERICK C. ALPERS ATTORNEYSCLOCK PU LSES PAIENIEDAPR 2 I974 3.801.981

SHEET 3 [IF 3 SYNCHRONIZING PULSE FROM MASTER STATION i 52 DETECTIONLATE GATED AMPLIFIER SBQ 4a as EARLY GATED DETECTION MEMORY AMPLIFIERAND ggggg IR IT 3O 32 I I v I I I I HIGH BISTABLE BISTABLE FREQUENCY AREACTANCE MULTIVIBRATOR MULTIVIBRATOR I OSCILLATOR CIRCUIT I 1 2 I I 0N1tOFF ON OFF I 34 4O 2 I I 4 2 REsET ADDITIONAL ADDITIONAL GEI I E R ATOR COUNTER I COUNTERI COUNTER 2 I (N) (8) (8) A I I I l 44 J I COUNTERTARGET RADAR ZERO TIME PULSE OR OTHER EVENTH 972% TRANSMITTER STOP TI MING sTART EVENT TIME BEAco N PuLsE ARRIVEE DATA OTHER STATIONS FIG. 4

FREDERICK C. ALPERS INVENTOR REMOTE CLOCK SYNCHRONIZATION SYSTEMSTATEMENT OF GOVERNMENT INTEREST The invention described herein may bemanufactured and used by or for the Government of the United States ofAmerica for governmental purposes without the payment of any royaltiesthereon or therefor.

SUMMARY OF THE INVENTION The present invention provides a system for theprecise synchronization of high frequency electronic clocks at stationsthat are remotely located from one another. In certain militaryapplications it is desirable to have remotely located clockssynchronized to within a few nanoseconds in order to measure the time ofarrival of microwave or other radiated signals at several differentstations, or to measure the propagation time between two or morestations. The present invention is concerned with relatively short cycleelectronic clocks (i.e., clocks with time events during a total cycle onthe order of one second as opposed to the normal 12 hours). The locationof these clocks may be in aircraft, missiles or other moving vehicles aswell as at fixed ground stations. One such location is selected as thecommand or master station and the remaining stations are identified asremote stations. It is required that the clocks of each of the remotestations be synchronized precisely with the clock at the master stationso that all clocks may begin timing events at precisely the same instant(accurate to within a few nanoseconds). With this synchronizationachieved the combination of precise starting and precise timingthereafter will permit events such as reception of a given enemy radarpulse at various aircraft to be measured in essentially absolute time,and corrections for clock differences are obviated. If thesynchronization process is repeated at frequent intervals, the need fora very large number of additional bits to identify the exact time of agiven event or time of arrival of a given pulse can be avoided.

Accordingly an object of the invention is to provide a remote clocksynchronization system that is both automatic and frequent so thathighly precise pulse time of arrival and event timing measurements canbe made without requiring extreme clock stability;

Another object of the invention is a provision of the remote clocksynchronization system in which synchronization and beacon pulses areused in a manner which permits many remote stations to be synchronizedto a given master station concurrently rather than in series;

A further object of the invention is the provision of self-synchronoustracking circuits to provide actual synchronization of clock oscillatorsso that event timing measurements are independent of the particulardistance between stations and any changes in distances.

Still another object of the invention is a provision of a remote clocksynchronization system in which the beacon pulses can serve a stationkeeping as well as a synchronization function.

Other objects and many of the attendant advantages of this inventionwill be readily appreciated as the same becomes better understood byreference to the following detailed description when considered inconnection with the accompanying drawings wherein:

FIG. 1 is a diagram of waveforms illustrating the principle ofoperation;

FIG. 2 is a modification of the diagram of FIG. 1 showing the insertionof a fixed delay (8);

FIG. 3 is a block diagram of a synchronization pulse timing controlcircuit for generating a synchronization pulse in accordance with theteaching of the invention;

FIG. 4 is a block diagram of a tracking circuit for synchronizing clockpulses at the remote station with the master station.

Referring to FIG. 1, a situation is shown in which the master stationand two remote stations, station A and station B, each transmit a pulseof electromagnetic energy at the instant of the start of a new clockcycle at that particular station. These pulses are hereinafteridentified as the beacon pulses for the respective stations. Inaddition, synchronization pulses A and B are transmitted by the masterstation at times prior to the master station beacon pulse. Ifsynchronization pulse A arrives at remote station A in exact coincidencewith transmission of the station A beacon pulse, and the station Abeacon pulse is received at the master station at a time such that theintervals d,, and (1,, are equal, then the station A beacon pulse mustoccur in exact coincidence with the master station beacon pulse, whichis the situation desired. Similarly, the station B beacon pulse willoccur in exact coincidence with master station beacon pulse if itstransmission is coincident with the arrival of synchronization pulse Band if d equals d The synchronization of many additional stations can besimilarly provided. In each case the two requisite conditions can beachieved through use of a selfsynchronous tracking circuit at the remotestation involved and suitable programming of the applicablesynchronization pulse at the master station. Means for obtaining thissuitable programming at the master station and the self synchronoustracking at the remote station will be taught in connection with FIG. 3and FIG. 4, respectively.

With the pulse timing arrangement illustrated in FIG. 1, practicalproblems in electromagnetically isolating a receiver from an adjacenttransmitter when both are .to operate simultaneously will necessitateuse of widely separated electromagnetic frequencies for the remotestation beacon pulse and the corresponding synchronization pulse fromthe master station. However, such use of widely separated frequencies isnot always acceptable, particularly in military environments. This leadsto the alternate pulse timing arrangement shown in FIG. 2, where a fixedinterval, 8, is inserted between the synchronization pulse and beaconpulse timings for each station. It is apparent that for the special caseof 6 0, the timing sequences of FIG. 2 become identical to those ofFIG. 1. Thus further discussion need be concerned only with the moregeneral situation shown in FIG. 2.

In FIG. 2, the station A beacon pulse will occur simultaneously with themaster station beacon pulse (and hence the clocks of the two stationswill be precisely synchronized) when both (1) the station Asynchronization pulse arrives at station A at a time that proceeds thestation A beacon pulse by the interval 8, and (2) the station A beaconpulse is received at the master station at a time such that interval dequals interval .d plus interval 8. Similarly, the station B clock willbe synchronized with the master station clock when both (1) the stationB synchronization pulse arrives at an interval 8 ahead of the station Bbeacon pulse, and (2) the interval d =d 8. In the present invention,conditions (1 above are met through the action of self-synchronoustracking circuits (to be described later) that are present at each ofthe remote stations. Conditions (2) are met by synchronization timingcontrol circuits located at the master station, with one such circuitpresent at the master station for each remote station that is to besynchronized.

The block diagram of a synchronization timing circuit is shown in FIG.3. A master clock generates both high rate clock pulses (e.g., one everyfour nanoseconds) and low rate zero time pulses (e.g., one approximatelyevery quarter of a second), where the ratio of the two rates is a largeintegral number, N, that remains constant (e.g., N 2 and determines theduration of a clock cycle. The clock pulses and the zero time pulses areeach fed both to time interval counter 12 and to a preset counter 14.Beacon pulses received from remote station A are also fed to timeinterval counter 12, and the connections are such that the zero timepulses are applied to the start input of counter 12 and the station Abeacon pulses are applied to the stop input. The output of counter 12 istherefore the counted number of clock pulses between zero time in agiven clock cycle and the reception of the station A beacon pulse inthat cycle. By reference to FIG. 2 it is apparent that this counteroutput number is a measure, in terms of basic clock pulse time units, ofthe time interval identified as d4 The output number from counter 12 isfed to digital adding circuit 16. A memory unit 18 also supplies anumber to adding circuit 16, which number is fixed with the design ofthe system and is equal to the selected interval 8 as measured in basicclock pulse time units (e.g., units of4 nanoseconds each). The output ofadding circuit 16 is therefore a number equivalent to the quantity (d8). This output is fed as a subtrahend to digital subtraction circuit20. Memory unit 18 feeds a fixed number as the minuend to subtractioncircuit 20, which number is N, the number of basic clock pulse timeintervals between the start of one clock cycle and the start of thefollowing cycle. The output of subtraction circuit 20 is therefore thequantity (N-d,1'-8) as measured in basic clock pulse time units. Thesubtraction circuit 20 output is connected as the numerical input thatsets preset counter 1 4.

With the inputs described above, preset counter 14 is constrained tocommence counting clock pulses at zero time in each counting cycle, andto cease counting and deliver an output pulse when a count of (N -d, -8)is reached. The output pulse from cogntg 1 4 then becomes thesynchronization pulse for remote station A and is relayed to thatstation by a suitable transmitter (not shown). In FIG. 2, this station Asynchronization pulse is shown to lead the master station beacon pulse(the zero time pulse) for the following cycle by a time interval that isidentified as d,,. Thus in terms of the original cycle, the timeinterval d, and the number N are defined in such ways that the station Asynchronization pulse appears after (Nd basic clock pulse time units inthe cycle, while the action of preset counter 14 and the other circuitsdescribed dictates that the station A synchronization pulse occurs atthe (N-d 8) point in the cycle. The circuit action is therefore onewhich forces which is precisely the condition (2) required for propersynchronization of remote station A to the master station. Identicalcircuits serve for remote station B and any additional remote stations.

The synchronization and tracking circuitry of each of the remotestations in shown in FIG. 4. The electronic clock 29 consists of a highfrequency oscillator 30, reactance circuit 32, pulse generator 34, andpreset counter 36. Oscillator 30 could typically be a crystal typeoperating at a frequency nominally the same as that of master clock 10in the master station (e.g., 250 MHz). Reactance circuit 32 should beone which changes the tuning of oscillator 30 very slightly inconsonance with a D.C. control voltage that is fed from memory circuit38. Pulse generator 34 should be a high frequency, very short pulse typethat generates a clock pulse for each cycle of the oscillator. Clockpulses will then occur at evenly spaced intervals for a few nanoseconds,which intervals should be nominally the same as the basic clock pulsetime intervals of the master station. Preset counter 36 could typicallybe of the binary type that counts clock pulses up to a predeterminedtotal N, which is the same as the N for the master station, and thenautomatically recycles itself to begin a new count. At each full-countor recycling point preset counter 36 delivers an output pulse for use inthe follow-on circuitry. The output pulse from clock preset counter 36is coupled to the first of two additional counters 40 and 42 that areconnected in series. Each of counters 40 and 42 introduce an additionalcount on the order of 2 to 2 clock pulses following the input pulsebefore producing an output pulse of its own. Counters 40 and 42 shouldbe of the non-recycling type and must wait for an input trigger pulsebefore they start counting. The delays between input and output that aregenerated in this manner are each made equal to the desired delay 8discussed above. The output of additional counter 42 becomes the zerotime pulse for this station and it is used both to trigger transmitter44 to generate the beacon pulse for this station and to start counter 46for event timing.

Synchronization of the remote station in such a way as to fulfillcondition (I) discussed above is effected through the use of early andlate gated amplifiers 48 and 50 that sense whether clock 29 tends tolead or lag the synchronizing pulses received from the master clock atinput terminal 52. Gating signals for gated amplifiers 48 and 50 aregenerated by bistable multivibrators 54 and 56, respectively, which inturn are synchronized by the counters 40 and 42 described above.Multivibrator 54 provides an early gate having a duration that commencesat the time of the output pulse from preset counter 36, and terminatesat the time of the output pulse from counter 40. Multivibrator 56provides a late gate of equal width that begins at the tennination ofthe early gate from multivibrator 54 and terminates at zero time in thefollowing clock cycle. The junction of the two gates therefore alwaysleads the zero time pulse of the station by 8. The tracking action ofthe remote station depends upon which of the gated amplifiers 48 and 50are gated on when the synchronizing pulse from the master stationarrives at input terminal 52. If the synchronizing pulse arrives duringthe interval of duration 6 while bistable multivibrator 54 and earlygated amplifier 48 are in the on condition, a pulse signal indication ofearly tracking results at the output of amplifier 48. Similarly, if thesynchronizing pulse from the master station arrives during the ensuinginterval of duration 8 when multivibrator 56 and gated amplifier 50 areon, a pulse signal indicative of late tracking occurs at the output ofamplifier 50. A balanced tracking condition will occur when the incomingpulse at terminal 52 arrives just as multivibrator 54 is turning off andmultivibrator 56 is turning on, which is the instant when the pulse atterminal 52 leads the local zero time by precisely the desired leadtime, 8. The remainder of the tracking loop consists of diodes 58 and 60which detect the early and late tracking signals, respectively, andmemory circuit 38 which feeds back the control voltage to reactancecircuit 32. Diodes 58 and 60 should be arranged in shunt and series,respectively, so as to add a positive increment to the memory circuitvoltage when the synchronization pulse coincides with the early gate,and a negative increment when coincidence is with the late gate. Diodes58 and 60 also 7 should be back-biased to provide isolation of memorycircuit 38 when no correction signals are present. Memory circuit 38 canbe simply a storage capacitor with a cathode follower type output. Withthe memory provided by memory circuit 38 and the self recyclingcapability of electronic clock 29 the tracking becomes self synchronous;that is, once a balanced tracking condition has been achieved, thecircuit of FIG. 4 will recycle itself in proper synchronization withoutalways requiring reception of a synchronizing pulse from the masterstation. At this point further tracking action is required only when thefrequency of high frequency 05- cillator 30 tends to drift apart fromthe frequency established by clock 10 in the master station. This selfsynchronous capability is of value in certain military situations wherethe reception of synchronization pulses from the master station may betemporarily disrupted due to aircraft motion, interferring signals, orthe like.

Obviously many modifications and variations of the present invention arepossible in the light of the above teachings. It is therefore to beunderstood that within the scope of the appended claims the inventionmay be practiced otherwise than as specifically described.

What is claimed is:

1. In a remote clock synchronizing system, the combination comprising:

a. a high frequency oscillator,

b. a pulse generator coupled to said oscillator for generating an outputpulse for each cycle of said oscillator,

c. a preset counter coupled to said pulse generator for generating anoutput pulse and recycling after a predetermined number of pulses arereceived from said pulse generator,

d. an input terminal for receiving synchronizing pulses from a remotelylocated master station,

e. a first multivibrator coupled to a preset counter for generating afirst gate signal so as to commence in response to the output pulse fromsaid preset counter and to terminate after a first predetermined timedelay.

f. a second multivibrator coupled to said preset counter for generatinga second gate signal so as to commence immediately upon termination ofsaid first gate signal and to terminate after a second predeterminedtime delay.

g. gate circuit means coupled to said input terminal and to said firstand second multivibrators and having an output coupled to saidoscillator for gating an output signal to increase the output frequencyof said oscillator when said first gate signal is in coincidence withsaid synchronizing pulse and an output signal to decrease the outputfrequency of said clock when said second gate signal is in coincidencewith said synchronizing pulse.

2. The synchronizing system of claim 1 wherein a memory circuit iscoupled between said gate circuit means and said oscillator to maintainthe adjusted frequency during the time interval between pulses receivedfrom said master station.

3. The synchronizing system of claim 1 wherein said second predeterminedtime delay is a counter circuit whose output provides the zero timepulse so that said time delay is accurately maintained.

4. The synchronizing system of claim 3 wherein said first and secondmultivibrators are bistable multivibrators, said first bistablemultivibrator being turned on by the output pulse from said presetcounter and being turned off by a first delayed signal initiated by theoutput pulse from said preset counter, said second bistablemultivibrator being turned on by said first delayed signal and beingturned off by a second delayed signal initiated by said first delayedsignal.

5. The synchronizing system of claim 4 wherein said gate circuit meansincludes:

a. a first gated amplifier for passing synchronizing pulses whenreceived in coincidence with the gate signal from said first bistablemultivibrator and a second gated amplifier for passing synchronizingpulses received in coincidence with the gate signal from said secondbistable multivibrator.

6. The system of claim 5 wherein said master station utilizes the timeof arrival of a beacon pulse from said remote station to generate asynchronization pulse such that the travel time of the master pulse tothe remote station is equal to the travel time of the remote stationbeacon pulse plus said second time delay and varying distances betweenstations are automatically accommodated.

1. In a remote clock synchronizing system, the combination comprising:a. a high frequency oscillator, b. a pulse generator coupled to saidoscillator for generating an output pulse for each cycle of saidoscillator, c. a preset counter coupled to said pulse generator forgenerating an output pulse and recycling after a predetermined number ofpulses are received from said pulse generator, d. an input terminal forreceiving synchronizing pulses from a remotely located master station,e. a first multivibrator coupled to a preset counter for generating afirst gate signal so as to commence in response to the output pulse fromsaid preset counter and to terminate after a first predetermined timedelay. f. a second multivibrator coupled to said preset counter forgenerating a second gate signal so as to commence immediately upontermination of said first gate signal and to terminate after a secondpredetermined time delay. g. gate circuit means coupled to said inputterminal and to said first and second multivibrators and having anoutput coupled to said oscillator for gating an output signal toincrease the output frequency of said oscillator when said first gatesignal is in coincidence with said synchronizing pulse and an outputsignal to decrease the output frequency of said clock when said secondgate signal is in coincidence with said synchronizing pulse.
 2. Thesynchronizing system of claim 1 wherein a memory circuit is coupledbetween said gate circuit means and said oscillator to maintain theadjusted frequency during the time interval between pulses received fromsaid master station.
 3. The synchronizing system of claim 1 wherein saidsecond predetermined time delay is a counter circuit whose outputprovides the zero time pulse so that said time delay is accuratelymaintained.
 4. The synchronizing system of claim 3 wherein said firstand second multivibrators are bistable multivibrators, said firstbistable multivibrator being turned on by the output pulse from saidpreset counter and being turned off by a first delayed signal initiatedby the output pulse from said preset counter, said second bistablemultivibrator being turned on by saiD first delayed signal and beingturned off by a second delayed signal initiated by said first delayedsignal.
 5. The synchronizing system of claim 4 wherein said gate circuitmeans includes: a. a first gated amplifier for passing synchronizingpulses when received in coincidence with the gate signal from said firstbistable multivibrator and a second gated amplifier for passingsynchronizing pulses received in coincidence with the gate signal fromsaid second bistable multivibrator.
 6. The system of claim 5 whereinsaid master station utilizes the time of arrival of a beacon pulse fromsaid remote station to generate a synchronization pulse such that thetravel time of the master pulse to the remote station is equal to thetravel time of the remote station beacon pulse plus said second timedelay and varying distances between stations are automaticallyaccommodated.